As a one-dimensional quantum confined material, colloidal semiconductor nanoplatelets have been widely studied as potential nonlinear materials due to their strong exciton effect and large two-photon absorption cross-section similar to that of two-dimensional materials. In this work, CdSe-CdS core-shell nanoplatelets were synthesized and third-order nonlinear optical properties related to shell thickness were measured using the Z-scan method. Measurement revealed a monotonic increase in the imaginary part of the third-order nonlinear susceptibility (Imχ(3)) of CdSe-CdS nanoplatelets, ranging from 0.62 × 10-13 esu to 2.43 × 10-13 esu, with the growth of shell thickness. The real part of the third-order nonlinear susceptibility (Reχ(3)) shows a non-monotonic change between 4.28 × 10-13 esu and 1.99 × 10-13 esu. The trends were further elucidated by analyzing the optical properties of the nanoplatelets, such as absorption, photoluminescence, and quantum yield, and understanding the variations in defect distribution, exciton binding energy, and quantum confinement effects. The results indicated that the appropriate passivation of the CdS shell effectively enhanced the luminescent performance and third-order nonlinearity of the nanoplatelets, while the induced defects and weakened quantum confinement effects due to the continued shell growth resulted in the opposite effect.